How to update database table schemas with NHibernate schema generation?

.net, fluent-nhibernate, nhibernate, orm, schema

Solution

The SchemaUpdate object provides database schema updating, by apparently generating and executing a series of SQL UPDATE statements (as well as constraint statements) when it's `void Execute(bool script, bool doUpdate)` function is called. The SchemaUpdate class is in the `NHibernate.Tool.hbm2ddl` namespace, which can be found in the Nhibernate.dll file.

SchemaUpdate is mentioned in chapter 15 of the nhibernate 1.0.2 toolset guide, here (section 15.1.5).

"The NHibernate FAQ" had (link now expired) a more complete example of how to use SchemaUpdate:

[Test]
public void Update_an_existing_database_schema()
{
    _cfg = new Configuration();
    _cfg.Configure();
    _cfg.AddAssembly(Assembly.LoadFrom("DataLayer.dll"));
    var update = new SchemaUpdate(_cfg);
    update.Execute(true, false);
}

Problem

I'm trying to figure out how to use NHibernate configuration with mapping to update table schemas, rather than dropping and recreating them. Currently I'm using the `NHibernate.Tool.hbm2ddl.SchemaExport` obj with FluentNHibernate to generate the database schema for a mysql database. While I can't say it's a huge problem, whenever I call `SchemaExport.Execute` on the database, it's going to drop all the tables and then recreate them. What would be way cooler is if I could just have it update the existing table structures retaining data where possible. But I don't really want to use a commerical product, or a code generator, because I don't like code generation in general, and I don't need this enough that I would consider paying for it. So hopefully any answer would keep these caveats in mind.

Original source